This invention relates to the granulation of materials and has particular relationship to the formation of free flowing solids consisting of substantially uniform spherical particles of ammonium nitrate and urea from melts of these materials. The spherical particles are called prills and the formation of prills from the melts is called prilling. Prills of ammonium nitrate and urea are used for agricultural fertilizer. Ammonium nitrate and urea are the principal types of nitrogen fertilizer in current large-scale commercial production.
This application concerns itself predominantly with prilling of ammonium nitrate and urea to form prills with respect to which it has unique advantages. It is to be understood that to the extent that this invention is adaptable to other materials and in other areas, such adaptation is within the scope of equivalents of this application and of any patent which may issue on or as a result thereof as scope of equivalents is interpreted and defined in Graver Tank & Manufacturing Co., v. Linde Air Products Co., 70 S Ct. 854 (1950).
In conventional fertilizer prilling operations, molten ammonium nitrate or urea is sprayed countercurrent to a stream of air in a relatively tall empty tower, and is cooled by the air and solidified and dried so that prills are produced. The air may be forced through the tower by fans, or natural draft may be employed to secure adequate air counterflow. In some prilling towers, auxiliary air may be locally in concurrent flow with the molten-particle flow; in this case there is also major air flow countercurrent to the particle flow.
Other methods of generating spherical solid particles include spheroidizers, pan granulators and fluidized bed techniques. In these latter methods, liquid fertilizer at relatively high temperatures is coated onto a mass of smaller particulates which are maintained in constant motion and are simultaneously cooled and dried by exposure to an air stream.
Spray nozzles of various types have been used to initially form and disperse the molten fertilizer into droplets. These include simple spray nozzles, comprised of planar or curved plates, containing many small holes or orifices, mechanically-driven centrifugal spray disks or buckets, and vibrating orifice plates. In all cases, however, molten material is fed to one or more such dispersion devices, so arranged as to disperse the molten drops more or less uniformly over the cross-sectional area of the prill tower.
In these high-temperature operations, including the prilling as well as the other methods, the ammonium nitrate or urea generates visible fumes which are a serious air-pollution problem. The postulated mechanisms involved in fume formation include both simple condensation of the sublimed vapors on cooling, and the possible recombination from the vapor phase of the chemical products resulting from dissociation.
In an article entitled, "Vapor Pressure of Ammonium Nitrate," published in the Journal of Chemical and Engineering Data, Vol. 7, No. 2, pages 227-228, April 1962, J. D. Brandner, N. M. Junk, J. W. Lawrence and J. Robins suggested that both solid and molten ammonium nitrate vaporize primarily by dissociation into ammonia and nitric acid. This reaction may be reversible and the theoretical back-reaction to ammonium nitrate from the cooler vapor phase may possibly form a submicron aerosol fume of ammonium nitrate solids. A parallel reaction may be written for the case of urea prilling, where dissociation again is from a hot melt or solution, to possible dissociation vapor products of ammonia and organic acids such as isocyanuric acid. These are conjectural mechanisms of fume formation, formulated to explain the apparent exponential increase in fume formation with increasing temperatures. It is not intended that their mention here should in any way affect this application or any patent which may issue on or as a result thereof.
In U.K. Pat. No. 1,208,850, H. E. Todd discloses the inhibition of fumes from hot ammonium nitrate vapor by adding ammonia to the air used to cool and/or dry or solidify hot solutions of ammonium nitrate. Todd states that "the desired amount of ammonia is injected into the inert gas stream before the gas contacts the ammonium nitrate." Brandner, et al. teaches that "by passing ammonia with nitrogen through a sample of ammonium nitrate . . . with both solid and liquid ammonium nitrate, the weight loss per liter of nitrogen passed through the sample is reduced to a fraction of its magnitude in the absence of ammonia."
The data presented by Todd in U.K. Pat. No. 1,208,850 on ammonium nitrate fume suppression by ammonia addition to air covers the range of 220.degree. F. to 277.degree. F. These data were presented in graphical form and show that, at 244.degree. F., a concentration of 0.083% by volume NH.sub.3 in air was required for fume suppression, while at 277.degree. F. ammonium nitrate temperature, a 75% reduction in fume level required a concentration of 0.29% NH.sub.3 by volume in air. The temperature range (to 277.degree. F.) covered by the Todd data is not characteristic or representative of the ammonium nitrate prilling temperatures employed commercially. Prilling of both ammonium nitrate and urea normally takes place industrially at temperature levels in excess of the melting points of these materials, which are 337.degree. F. and 271.degree. F., respectively. Industrial prilling temperatures for ammonium nitrate are generally in the range of 345.degree. F. to 380.degree. F., and at these temperatures, the concentration of NH.sub.3 in air required for any significant fume suppression becomes uneconomically large. For example, it has been estimated that for an ammonium nitrate temperature of 380.degree. F. an NH.sub.3 concentration of 1.5% would be required for 80% fume suppression, and 6% NH.sub.3 concentration for 90% suppression. For a typical ammonium nitrate prilling tower producing 1000 tons/day of prills, using 200,000 CFM of forced air flow, the 6% by volume NH.sub.3 requirement is equal to 17 tons/hr. of NH.sub.3. To avoid losing this much NH.sub.3 to the exhaust air, the NH.sub.3 would have to be scrubbed out of the exhaust air. If HNO.sub.3 solution is used for absorption, a minimum of 63.2 tons of HNO.sub.3 per hour is needed to neutralize the NH.sub.3. This would be equivalent to manufacturing more than 80 tons/hr. of ammonium nitrate in the exhaust air scrubber, or 1925 tons/day, which is almost double the amount of the initial plant capacity. At 1.5% NH.sub.3 by volume, corresponding to 80% fume suppression, 8500 lbs./hr. of NH.sub.3 would be needed, as would 15.8 tons of HNO.sub.3 /hr. for neutralization. It is therefore clear that the NH.sub.3 feed rates and the associated scrubbing loads and limits that are needed to suppress fume formation in the prilling operation by the method of Todd are impractical and uneconomic at temperatures in excess of the prill material melt point, which elevated temperatures are invariably and necessarily used in prilling.
It is an object of this invention to overcome the drawbacks and disadvantages of the above-described prior art and to provide prilling apparatus in whose use fumes shall be suppressed economically by use of ammonia in relatively low quantities.
In accordance with this invention, prilling apparatus is provided which establishes and maintains a quiescent zone of pure, or highly concentrated, NH.sub.3 directly below and in contact with the molten ammonium nitrate or urea dispersion device or spray nozzle orifices forming the spray. The spray particles are formed in, and initially fall through an NH.sub.3, or NH.sub.3 -rich, gas zone. This relatively stagnant ammonia atmosphere is formed and contained under the spray head by means of an outer shroud or bell whose skirt extends below and contiguous to the bottom of the molten fertilizer spray nozzle or dispersion device. Ammonia gas or an atmosphere enriched in ammonia and containing more than 50% by volume ammonia is fed to this shroud or bell. Although any ammonia-rich gas containing more than 50% by volume may be employed, particularly if a waste gas of this composition is available, it is preferable to use ammonia gas of 95 to 100% ammonia concentration. Gas streams of the latter composition are normally available in a fertilizer plant at super-atmospheric pressures, or alternately, are easily generated from liquid ammonia. Because ammonia is less dense than air, the shrouding of the molten material spray nozzle provides for the establishment of the desired stable zone of relatively quiescent ammonia by trapping the ammonia under the shroud by reason of density differential relative to the air outside the shroud. Additionally, the ammonia trapped under the shroud is continuously heated by the molten prill and this augments the inherent molecular weight gas density difference relative to air. The ammonia is concentrated in the hottest region directly under the spray nozzle where the hottest material is emitted and is thus heated to a higher degree than the air below or outside the shroud.
In some prilling towers, a plurality of spray nozzles or dispersion devices, as many as 20 or more, may be provided. In such towers, a single shroud may be provided for the assembly of spray nozzles or dispersion devices, or several shrouds may be provided, each for several of the spray nozzles or dispersion devices. A single shroud is provided in prilling towers which have a single central spray nozzle or dispersion device.
Whether fume formation occurs because of dissociation/recombination, vapor pressure, sublimation/condensation, or other mechanisms, the tendency to form fumes increases with increasing temperature. By whatever suppression mechanism, exposure of the hottest ammonium nitrate or urea melt to the pure or enriched ammonia atmosphere completely suppresses fume formation at this zone. The quantity of ammonia required for accomplishing total fume suppression by use of a trapped quiescent atmosphere is small compared to the flow of countercurrent cooling prill tower air. It is also small relative to the amount of ammonia required for the air dilution method of Todd. The latter method presents a diluted low-concentration gas stream uniformly to the spray over its entire drop path in the prill tower, over which the spray temperature varies from molten liquid temperatures as high as 380.degree. F. at the spray header to 200.degree. F. at the bottom of the tower. It has been realized in arriving at this invention that because the ammonia-air concentration requirements for fume suppression vary with spray temperature, with the maximum ammonia concentrations being required for the hottest spray, Todd's air dilution method uses uneconomical and excessive amounts of ammonia to achieve suppression for the hottest top portion of the tower. Alternately, Todd's method accomplishes incomplete fume suppression at lower average ammonia concentrations which inhibit fume formation in the lower and cooler portion of the prill tower, but not at the hotter upper regions.
In the practice of this invention, the hottest spray is exposed to the highest ammonia concentration. In falling through and out of the trapped zone of ammonia created in accordance with this invention, the spray particles carry with them a laminar and stagnant boundary layer of ammonia. The particles are thus "coated" or enveloped by a gaseous surface layer of ammonia. This phenomenon is similar to that encountered in high-diving into water, wherein the diver on entering the water carries with him an entrained layer of air. The concentration of ammonia in the laminar sublayer adjacent to the cooling prill surface is diminished only by the molecular diffusion of ammonia into the surrounding air zone, which is a slow process, and by convective heat transfer. However, as the spray cools, the ammonia concentration required for fume suppression decreases, so the two effects of loss of ammonia with distance of fall of spray and decreasing temperature serve to counterbalance each other. The acceleration of the cooling of the spray is an additional advantage of the invention.